Processing a three-dimensional representation of a scene

Abstract

A method of processing a three dimensional representation of a scene 31 via identification of one or more points within that representation as background points based on distance 32 from a viewing zon

Description

Field of the Disclosure The present disclosure relates to methods, systems, and apparatuses for processing a three-dimensional representation of a scene. Background to the Disclosure Three-dimensional representations of environments are used in many contexts, including for the generation of virtual reality videos, in which depth information for a plurality of points of the representation is used to generate different images for a left eye and a right eye of a user. Typically, substantial processing power is required to determine such a three-dimensional representation, and the file size of files associated with these representations is typically large so that substantial amounts of storage are needed to keep the files and substantial amounts of bandwidth are required to transfer the files. Summary of the Disclosure According to an aspect of the present disclosure, there is described: a method of processing a three-dimensional representation of a scene, the method comprising: identifying a three-dimensional representation of a scene; identifying one or more points in the three-dimensional representation as background points based on a distance of said points from a viewing zone associated with the representation; processing the identified points, preferably so as to associate these points with a two-dimensional background image. Preferably, the two-dimensional background image comprises a monoscopic image. Preferably, the method comprises generating a two-dimensional immersive image based on the three-dimensional representation and the two-dimensional background image. Preferably, the method comprises generating a two-dimensional immersive image for each eye of a viewer. Preferably, each of the two-dimensional immersive images is based on the same two-dimensional background image. Preferably, the method comprises determining a distance the identified points from a viewing zone associated with the scene; and associating the points with a monoscopic two-dimensional image in dependence on the distance exceeding a threshold distance. Preferably, the method comprises determining a distance the identified points from a viewing zone associated with the scene; and associating the points with a stereoscopic two-dimensional image in dependence on the distance not exceeding a threshold distance. Preferably, processing the identified points comprises one or more of: removing the points from the three-dimensional representation; modifying the three-dimensional representation to include a reference to the background image; replacing an attribute value of the identified points with a reference to the background image; and generating the two-dimensional background image based on the identified points. Preferably, the two-dimensional background image comprises a background of the scene. Preferably, the two-dimensional background image comprises a 360-degree image. Preferably, the method comprises: determining a distance of the identified points from a viewing zone associated with the scene; and determining the two-dimensional background image based on the distance. Preferably, the method comprises determining a resolution of the two-dimensional background image based on the distance. Preferably, the method comprises rendering a two-dimensional image based on the three-dimensional representation and combining the rendered two-dimensional image with the background image based on the locations of the background points. Preferably, the method comprises forming the two-dimensional background image based on attribute values of the identified points. Preferably, the method comprises forming the two-dimensional background image at a first device and transmitting the two-dimensional background image to a second device. Preferably, the method comprises receiving the background image and the three-dimensional representation from different databases and / or computer devices. Preferably, the method comprises: identifying a plurality of two-dimensional background images associated with the scene; and determining a two-dimensional immersive image based on the plurality of identified two-dimensional background images. Preferably, determining the two-dimensional immersive image comprises: rendering one or more two-dimensional objects based on the three-dimensional representation; and superimposing the two-dimensional objects onto the two-dimensional background image. Preferably, the two-dimensional background image is associated with a location in the three-dimensional representation. Preferably, the two-dimensional background image is associated with one or more points of the three-dimensional representation, preferably with a location of said one or more points. Preferably, the method comprises: identifying the one or more points of the three-dimensional representation based on a characteristic of said points, preferably based on a distance of said points from a viewing zone of the scene; and combining the two-dimensional image with the three-dimensional representation based on the identified points. Preferably, the method comprises locating the two-dimensional image within the scene based on the locations of the identified points. Preferably, the method comprises: identifying a point of the three-dimensional representation; identifying a characteristic of the point; and modifying the point based on the characteristic. Preferably, modifying the point comprises replacing the point with a two-dimensional background image and / or a reference to a two-dimensional background image. Preferably, the method comprises: identifying a point of the three-dimensional representation; identifying a characteristic of the point; and modifying the point based on the characteristic; wherein modifying the point comprises replacing the point with a two-dimensional background image and / or a reference to a two-dimensional background image. According to another aspect of the present disclosure, there is described a method of processing a three-dimensional representation of a scene, the method comprising: identifying a point of the three-dimensional representation; identifying a characteristic of the point; and modifying the point based on the characteristic; wherein modifying the point comprises replacing the point with a two-dimensional background image and / or a reference to a two-dimensional background image. Preferably, the characteristic comprises a distance of the point from a viewing zone of the scene. Preferably, the method comprises replacing the point based on the distance exceeding a distance threshold. Preferably, the characteristic comprises a difference between a left eye attribute of the point and a right eye attribute of the point. Preferably, the characteristic is associated with the point being static. Preferably, the method comprises replacing the point based on an indicator of the point being static exceeding a threshold value. Preferably, the method comprises: identifying a first point in a three-dimensional representation of a scene; identifying a second point in a second three-dimensional representation of a scene; and determining that the first point and / or the second point is a static point based on the locations and / or attributes of the first point and the second point. Preferably, the method comprises determining that the first point and / or the second point is a static point based on one or more of: determining that the first point and the second point have similar locations; and determining that the first point and the second point have similar attributes. Preferably, the method comprises determining that the first point and / or the second point is a static point based on the locations and / or attributes of corresponding points in a plurality of three-dimensional representations (e.g. consecutive three-dimensional representations), preferably at least 3 representations, at least 5 representations, at least 10 representations, and / or at least 20 representations. Preferably, the two-dimensional image is a frame of a video. Preferably, the three-dimensional representation is associated with a frame of a video. Preferably, the method comprises: processing (e.g. decoding) the three-dimensional representation using a first hardware device; and processing (e.g. decoding) the two-dimensional background image using a second hardware device. According to another aspect of the present disclosure, there is described a method of processing a three-dimensional representation of a scene, the method comprising: identifying a three-dimensional representation of a scene; determining a two-dimensional background image associated with the three-dimensional representation of the scene; and generating a two-dimensional immersive image based on the three-dimensional representation and the two-dimensional background image. Preferably, the method comprises: identifying one or more background points in the three-dimensional representation; and determining a two-dimensional background image associated with the background points. Preferably, the method comprises generating a two-dimensional immersive image for each eye of a viewer. Preferably, the method comprises rendering the two-dimensional background image based on a viewpoint of a viewer of the immersive image. Preferably, the two-dimensional background image comprises a tiled image and the method comprises rendering a tile of the two-dimensional background image based on a viewpoint of a viewer of the immersive image. Preferably, determining the two-dimensional background image comprises one or more of: determining the two-dimensional background image based on a background point of the three-dimensional representation, preferably based on an attribute value of the background point; and extracting the two-dimensional background image from a bitstream containing the three-dimensional representation. Preferably, the three-dimensional representation is associated with a viewing zone, the viewing zone comprising a subset of the scene and / or the viewing zone enabling a user to move through a subset of the scene. Preferably, the user is able to move within the viewing zone with six degrees of freedom (6DoF). Preferably, the viewing zone has a volume of less than 50% of the volume of the scene, less than 20% of the volume of the scene, and / or less than 10% of the volume of the scene. Preferably, the viewing zone has, or is associated with, a volume, preferably a real-world volume, of less than five cubic metres (5m3), less than one cubic metre (1 m3), less than one-tenth of a cubic metre (0.1 m3) and / or less than onehundredth of a cubic metre (0.01 m3). Preferably, the three-dimensional representation comprises a point cloud. Preferably, the method comprises storing the three-dimensional representation and / or outputting the three-dimensional representation. Preferably, the method comprises outputting the three-dimensional representation to a further computer device. Preferably, the method comprises generating an image and / or a video based on the three-dimensional representation. Preferably, the method comprises forming one or more two-dimensional representations of the scene based on the three-dimensional representation. Preferably, the method comprises forming a two-dimensional representation for each eye of a viewer. Preferably, the point is associated with one or more of: a location; an attribute; a transparency; a colour; and a size. Preferably, the point is associated with an attribute for a right eye and an attribute for a left eye. Preferably, the scene comprises one or more of: an extended reality (XR) scene; a virtual reality (VR) scene; an augmented reality (AR) scene; and a mixed reality (MR) scene. Preferably, the method comprises forming a bitstream that includes the three-dimensional representation and / or the immersive two-dimensional image. According to another aspect of the present disclosure, there is described a system for carrying out the aforesaid method, the system comprising one or more of: a processor; a communication interface; and a display. According to another aspect of the present disclosure, there is described an apparatus for processing a three-dimensional representation of a scene, the apparatus comprising: means for (e.g. a processor for) identifying a three-dimensional representation of a scene; means for (e.g. a processor for) identifying one or more points in the three-dimensional representation as background points based on a distance of said points from a viewing zone associated with the representation; and means for (e.g. a processor for) processing the identified points, preferably so as to associate these points with a two-dimensional background image. According to another aspect of the present disclosure, there is described an apparatus for processing a three-dimensional representation of a scene, the apparatus comprising: means for (e.g. a processor for) identifying a point of the three-dimensional representation; means for (e.g. a processor for) identifying a characteristic of the point; and means for (e.g. a processor for) modifying the point based on the characteristic; wherein modifying the point comprises replacing the point with a two-dimensional background image and / or a reference to a two-dimensional background image. According to another aspect of the present disclosure, there is described an apparatus for processing a three-dimensional representation of a scene, the apparatus comprising: means for (e.g. a processor for) identifying a three-dimensional representation of a scene; means for (e.g. a processor for) determining a two-dimensional background image associated with the three-dimensional representation of the scene; and means for (e.g. a processor for) generating a two-dimensional immersive image based on the three-dimensional representation and the two-dimensional background image. According to another aspect of the present disclosure, there is described a bitstream comprising one or more points processed using the aforesaid method. According to another aspect of the present disclosure, there is described a bitstream associated with a three-dimensional representation of a scene, the bitstream comprising one or more background points of the three-dimensional representation, the background points each being associated a two-dimensional background image. Preferably, the bitstream comprises one or more flags that indicate one or more of: whether the three-dimensional representation contains one or more background points; a feature, e.g. resolution or a size, of a two-dimensional background image associated with one or more background points; a location of a repository that contains a two-dimensional background image; and a process by which the two-dimensional background images should be combined with the initial two-dimensional immersive images. Preferably, the bitstream comprises the two-dimensional background image. Preferably, the bitstream comprises: a first section associated with, e.g. containing points of, the three-dimensional representation; and a second section associated with, e.g. containing, one or more two-dimensional background images. Preferably, bits signalling two-dimensional background images are interlaced with bits signalling points of the three-dimensional representation. According to another aspect of the present disclosure, there is described an apparatus (e.g. an encoder) for forming and / or encoding the aforesaid bitstream. According to another aspect of the present disclosure, there is described an apparatus (e.g. a decoder) for receiving and / or decoding the aforesaid bitstream. Any feature in one aspect of the disclosure may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, features implemented in hardware may be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the disclosure can be implemented and / or supplied and / or used independently. The disclosure also provides a computer program and a computer program product comprising software code adapted, when executed on a data processing apparatus, to perform any of the methods described herein, including any or all of their component steps. The disclosure also provides a computer program and a computer program product comprising software code which, when executed on a data processing apparatus, comprises any of the apparatus features described herein. The disclosure also provides a computer program and a computer program product having an operating system which supports a computer program for carrying out any of the methods described herein and / or for embodying any of the apparatus features described herein. The disclosure also provides a computer readable medium having stored thereon the computer program as aforesaid. The disclosure also provides a signal carrying the computer program as aforesaid, and a method of transmitting such a signal. The disclosure extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings. The disclosure will now be described, by way of example, with reference to the accompanying drawings. Description of the Drawings Figure 1 shows a system for generating a sequence of images. Figure 2 shows a computer device on which components of the system of Figure 1 may be implemented. Figure 3 shows a method of determining a three-dimensional representation of a scene. Figures 4a and 4b show method of determining a point based on a plurality of sub-points. Figure 5 shows a scene comprising a viewing zone. Figures 6a and 6b show arrangements of capture devices for determining points of the three-dimensional representation. Figure 7 shows different versions of a point that may be captured by different capture devices. Figures 8a and 8b shows grids formed by the different capture devices. Figure 9 shows an arrangement for indicating an angle of a point from a capture device used to capture the point. Figure 10 shows a method of determining a two-dimensional image based on a three-dimensional representation. Figure 11 shows a method of modifying a point of a three-dimensional representation. Figure 12 shows a method of determining a static point of a three-dimensional representation. Figure 13 shows a bitstream. Description of the Preferred Embodiments Referring to Figure 1, there is shown a system for generating a sequence of images. This system can be used to generate, and then display, a representation of an environment, which may comprise a VR environment (or an XR environment). The system comprises an image generator 11, an encoder 12, a transmitter 13, a network 14, a receiver 15, a decoder 16 and a display device 17. These components may each be implemented on separate apparatuses. Equally, various combinations of these components may be implemented on a shared apparatus; for example, the image generator 11, the encoder 12, and the transmitter 13 may all be part of a single image data generation device. Similarly, the receiver 15, the decoder 16, and the display device 17 may all be a part of a single image rendering device. Typically, the system comprises at least one encoding computer device (e.g. a server of a content provider) and at least one rendering computer device (e.g. a VR headset). Referring to Figure 2, each of the components, and in particular the image generator 11, the encoder 12, the transmitter 13, the receiver 15, the decoder 16 and the display device 17 is typically implemented on a computer device 20, where, as described above, a plurality of these components may be implemented on a shared computer device. Each computer device comprises one or more of: a processor 21 for executing instructions (e.g. so as to perform one or more of the steps of the various methods described below), a communication interface 22 for facilitating communication between computer devices (e.g. an ethernet interface, a Bluetooth® interface, or a universal serial bus (UBS) interface, a memory 23 and / or storage 24 for storing information and instructions (e.g. a random access memory (RAM), a read only memory (ROM), a hard drive disk (HDD) a solid state drive (SSD), and / or a flash memory, and a user interface 25 (e.g. a display, a mouse, and / or a keyboard) for enabling a user to interact with the computer device. These components may be coupled to one another by a bus 25 of the computer device. The computer device 20 may comprise further (or fewer) components. In particular, the computer device (e.g. the display device 17) may comprise one or more sensors, such as an accelerometer, a GPS sensor, or a light sensor. These sensors typically enable the computer device to identify an environmental condition and / or an action of wearer of the display device. Turning back to Figure 1, the image generator 11 is configured to generate a sequence of image data (e.g. a sequence of image frames) to enable the display device 17 to use this image data to display a plurality of images. The image data may comprise one or more digital objects and the image data may be generated or encoded in any format. For example, the image data may comprise point cloud data, where each point has a 3D position and one or more attributes. These attributes may, for example, include, a surface colour, a transparency value, an object size and a surface normal direction. Each attribute may have a value chosen from a continuous range or may have a value chosen from a discrete set. The image data enables the later rendering of images. This image data may enable a direct rendering (e.g. the image data may directly represent an image). Equally, the image data may require further processing in order to enable rendering. For example, the image data may comprise three-dimensional point cloud data, where rendering a two-dimensional image using this data requires processing based on a viewpoint of this two-dimensional image. The image data may comprise depth map data, where one or more pixels or objects in the image is associated with a depth that is specified by the depth map data. The depth map data may be provided as a depth map layer, separate from an image layer. In some contexts, such as MPEG Immersive Video (MIV), the image layer may instead be described as a texture layer. Similarly, in some contexts, the depth map layer may instead be described as a geometry layer. The image data may include a predicted display window location. The predicted display window location may indicate a potion of an image that is likely to be displayed by the display device 17. The predicted display window location may be based on a viewing position (such as a virtual position and / or orientation of the user in a 3D environment) of the user, where this viewing position may be obtained from the display device. The predicted display window location may be defined using one or more coordinates. For example, the predicted display window location may be defined using the coordinates of a corner or center of a predicted display window, and may be defined using a size of the predicted display window. The predicted display window location may be encoded as part of metadata included with the frame. The image data for each image (e.g. each frame) may include further information, which may be provided as a part of an image, e.g. as part of the point cloud data, or as separate layers. In particular, the image data may include audio information or haptic feedback information indicating audio or haptics which can accompany displayed visual data. An audio layer or haptic layer may accompany each image, and may be omitted for images where no accompanying audio or haptics are required. Similarly, the image data may comprise interactivity information, where the image data may contain or indicate elements with which a user can interact. The interactivity information may, for example, define a behaviour of an element, where a user is able to interact with the element based on this behaviour. The behaviour typically defines a change in an element that occurs as a result of a user interaction where this change may comprise a change in the attributes of the element or in the rendering of the element. As an example, where an image contains a target element, the target element may be arranged to disappear when a user interacts with this element, or to provide feedback indicating that the user has interacted with the target. This interactivity data may be provided as part of, or separately to, the image data. The image data may indicate, or may be combinable with, a state of the virtual environment, a position of a user, or a viewing direction of the user. Here, the position and viewing direction may be physical properties of the user in the real-world, or position and viewing direction may also be purely virtual, for example being controlled using a handheld controller. The image generator 11 may, for example, obtain information from the display device 17 that indicates the position, viewing direction, or motion of the user. Equally, the image generator may generate image data such that it can later be combined with this position, viewing direction, or motion, where the image generator may generate a full scene which is only partially viewed by a user depending on the position of that user. In some cases, the generated image may be independent of user position and viewing direction. This type of image generation typically requires significant computer resources such as a powerful GPU, and may be implemented in a cloud service, or on a local but powerful computer. For example, a cloud service (such as a Cloud Rendering Service (CRN)) may reduce the cost per-user and thereby make the image frame generation more accessible to a wider range of users. Here “rendering” refers at least to an initial stage of rendering to generate an image. Further rendering may occur at the display device 17 based on the generated image to produce a final image which is displayed. The image generator 11 may, for example, comprise a rendering engine for initially rendering a virtual environment such as a game or a virtual meeting room. The encoder 12 is configured to encode frames to be transmitted to the display device 17. The encoder may be implemented using executable software or may be implemented on specific hardware such as an ASIC. In some embodiments, the image generator 11 may transmit raw, unencoded, data through the network 14. However, such transmission typically leads to a high file size and requires a high bandwidth so that it is typically desirable to encode the data prior to the transmission. The encoder 12 may encode the image data in a lossless manner or may encode the data a lossy manner. The encoder may apply inter-frame or intra-frame compression based on a currently-encoded frame and optionally one or more previously encoded frames. The encoder may be a multi-layer encoder, such as a low complexity enhancement video codec (LCEVC) enabled encoder. Where the generated frames comprise depth map data, the encoder 12 may perform layered encoding on each instance of image data (e.g. each frame) to generate an encoded frame comprising a base depth map layer and an enhancement depth map layer. Encoding a depth map in this way may improve compression. In some applications, such as HDR video, depth maps are desirably highly detailed with a bit depth of up to twelve or fourteen bits, which is a significant increase in the data to be transmitted. As a result, providing ways to improve compression of the depth map can make more realistic depth map-based displays viable when performing rendering or transmission of rendered data in real-time. Furthermore, this type of layered encoding makes it easy to drop (and then pick back up) one or more of the layers, which provides flexibility and tools for bandwidth management. Layered encoding is also helpful as the final decoder / user device (such as a user display device) can choose whether to process these extra layers. For example, in a non-layered approach, the best the end device (i.e. the receiver, decoder or display device associated with a user that will view the images) can do is determine that it does not have enough resources for a given quality (be it resolution, frame rate, inclusion of depth map) and then signal to the controller / renderer / encoder that it does not have enough resources. The controller then will send future images at a lower quality. In that alternative scenario, the end device still unfortunately has to process the higher quality data until the lower quality data arrives, if it can process the received images at all. In some of the described embodiments, this situation is improved upon because when / if the end device determines for example that it does not have the processing capabilities to handle the highest level of quality, then it can drop and / or choose not to process certain layers. The end device may also signal to the controller that it needs a lower level of quality, but in the meantime the end device can only process the number of layers that it can handle. Therefore, the end device can react to conditions much more quickly. In some cases, depth map data may be embedded in image data. In this case, the base depth map layer may be a base image layer with embedded depth map data, and the enhancement depth map layer may be an enhancement image layer with embedded depth map data. Alternatively, when the generated images comprise a depth map layer separate from an image layer and multi-layer encoding is applied, the encoded depth map layers may be separate from the encoded image layers. This has the advantage that the encoded depth map layers can be dropped under some conditions while still retaining image layers that can be displayed (albeit with a lower level of realism). For example, the encoded depth map layers can be dropped by a transmitter or encoder when available communication resources are reduced, or can be dropped by an end device which lacks the processing resources to handle the highest level of quality. Similarly, if some images comprise an audio base layer, a haptic feedback base layer, an audio enhancement layer or a haptic feedback enhancement layer, these can be processed or dropped flexibly. Again similarly, if some images comprise an interactivity data base layer or an interactivity enhancement layer these can be processed or dropped flexibly. For example, certain interactions may only be possible where a threshold bandwidth is available, where complex interactions (e.g. those enabling a conversation with a digital object) may be disabled before less complex interactions (e.g. changing a pixel colour) are disabled. Additionally or alternatively, where the image data comprises point cloud data, the encoder may apply a point cloud data encoding technique such as described in European patent application EP21386059.6, which is incorporated herein by reference. Such a point cloud encoder may act as a base encoder for a layered encoding technique such as LCEVC or VC-6. Notably LCEVC and VC-6 techniques encode and decode a layered signal, but are agnostic about the content type of data encoded in the signal. For example, the signal can include textures, video frames, geometry or depth data, meshes, point clouds, rendering attributes or physics engine attributes. The transmitter 13 may be any known type of transmitter for wired or wireless communications, including an Ethernet transmitter or a Bluetooth transmitter. The transmitter 13 may be configured to make decisions about how to transmit the image data, and / or may provide feedback to the encoder 12 or the image generator 11. For example, the transmitter may determine available communication resources (e.g. bandwidth) for transmitting image data, and may drop one or more layers from an encoded frame, or indicate to the image generator and / or encoder that image data should be generated and encoded with fewer layers, when insufficient bandwidth is available for transmission of all generated data. As specific examples, the transmitter may be configured to drop a depth map layer, an LCEVC enhancement layer, or a VC-6 enhancement layer from a frame when insufficient communication resources are available. The network 14 provides a channel for communication between the transmitter 13 and the receiver 15, and may be any known type of network such as a WAN or LAN or a wireless Wi-Fi or Bluetooth network. The network may further be a composite of several networks of different types. Many users only have access to a network with a bandwidth of 30MBps which can lead to latency jitter when streaming. The required bandwidth and the observed latency can be reduced by means of tactics such as forward-looking rendering and last-millisecond reprojection, which are enabled by improved compression. The receiver 15 may be any known type of receiver for wired or wireless communications, including an Ethernet transmitter or a Bluetooth transmitter. The decoder 16 is configured to receive and decode an encoded frame. The decoder may be implemented using executable software or may be implemented on specific hardware such as an ASIC. The display device 17 may for example be a television screen or a VR headset. The timing of the display may be linked to a configured frame rate, such that the display device may wait before displaying the image. The display device may be configured to perform warping, that is, to obtain a final display window location, adjust a warpable image to obtain a final image corresponding to a final viewing direction of the user, and display the final image. In this regard, the image data is typically arranged to provide a warpable image for which a portion of the image that is displayed at the display device 17 is dependent on a position or orientation of a viewer. The warpable image may then be rendered before a most up to date viewing direction of the user is known. The warpable image may be transmitted to the display device, or the warpable image may be transmitted to a rendering node which is near to the display device, and the display device or rendering node may perform time warping to generate a displayed image portion based on the warpable image and the most up to date viewing direction of the user. As mentioned above, a single device may provide a plurality of the described components. For example, a first rendering node may comprise the image generator 11, encoder 12 and transmitter 13. Additional similar rendering nodes may be included in the system, and may work together to generate the sequence of frames. In one case, multiple rendering nodes may each provide separate image data to an image data assembling node; for example, each rendering node may provide a part of a sequence of frames to a frame assembling node. For example, the receiver 15, decoder 16 or display device 17 may be configured to assemble parts of image data from multiple sources to generate a sequence of images for display on the display device. Alternatively, the image data assembling node may be separate from the receiver 15, decoder 16 and display device 17. Additionally or alternatively, multiple rendering nodes may be chained. In other words, successive rendering nodes may add to a sequence of image data as it passes from rendering node to rendering node, and eventually a complete sequence of image data is then provided to the receiver 15. Furthermore, each rendering node may obtain components of a render from multiple upstream rendering nodes and / or distribute components of a render to multiple downstream rendering nodes. A chain of rendering nodes may be useful for performing different rendering tasks that require different quantities of processing resources, or different frame rates. For example, a company may provide distributed processing in the form of a centralised hub which has abundant processing resources but is distant from users, and peripheral locations which have more scarce processing resources but are closer to users. Expensive but fairly static rendering features such as background lighting or environmental impact on sound may be generated at the central hub (for example using ray tracing), while features that require fewer resources but faster responses or higher frame rates may be generated closer to the user. In other words, the more responsive a rendering feature needs to be, the lower latency it needs between the rendering node which generates the feature and the user display and, in a chain of rendering nodes, the node which generates each rendering feature can be chosen based on a required maximum latency of that feature. On the other hand, if it is expensive to generate a rendering feature, then it may be preferable to generate the feature less frequency and with a higher maximum latency. For example, a static, high-quality background feature may be generated early in the chain of rendering nodes and a dynamic, but potentially lower-quality, foreground feature may be generated later in the chain of rendering nodes, closer to the user device. Here, environmental impact on sound means, for example, a set of surfaces may be constructed where each surface has different sound reflection and absorption properties depending upon material and shape. The frame rates may be matched by creating multiple frames with features generated at the lower frame rate, and combining them with the frames with features generated at the higher frame rate. In a nonlimiting embodiment, a preliminary rendering generates volumetric object data including motion vectors at a first (lowest) frame rate, then produces 2D rendered frames plus depth information for a specific user at a second (higher) frame rate, then transmits video plus depth data to the user device, which produces final frames for display via space warping (depth-based reprojections) at a third (highest) frame rate. One or more of these steps may be performed in combination with the other described embodiments. The viewing position of the user may change as additional rendering tasks are performed at different rendering nodes in the chain. Each or any rendering node may obtain an updated viewing position before performing its respective rendering task. Additionally, the system may simultaneously generate multiple sequences of image data for different respective users or different respective display devices. For example, in the context of a VR or AR experience, each user or display device may view a different 3D environment, or may view different parts of a same 3D environment. When using a chain of rendering nodes, each node may serve multiple users or just one user. For example, a starting rendering node (e.g. at a centralised hub) may serve a large group of users. For example, the group of users may be viewing nearby parts of a same 3D environment. In this case, the starting node may render a wide zone of view (“field of view”) which is relevant for all users in the large group. The starting node may send this wide field of view to a first middle rendering node which renders additional aspects of the 3D environment. These additional aspects may for example be aspects which require less processing power to render, or may be aspects which are specific to individual users of the group. Additionally, the middle rendering node may render features in a smaller field ofviewthan the starting node - this smaller field of view may be relevant to each user rather than the group of users. The first middle rendering node may additionally only serve a smaller number of users (e.g. half of the large group of users), with the remaining users being served by a second middle rendering node which also receives the wide field of view from the starting node. The middle rendering node(s) may then send sequences of second partially or fully rendered frames to an end device for each user. The end device may perform further processes such as warping or focal distance adjustments, optionally using depth map data. Preferably, each rendering node encodes the partially or fully rendered frames before transmitting them on to a next rendering node or to the receiver 15. This means that the required communication resources can be reduced when the rendering nodes are separated by one or more networks, or more generally are implemented in a distributed system such as a cloud. However, each rendering node in a chain is encoding a different partially or fully rendered frame, with different data. Therefore, it may be advantageous for different rendering nodes to use different rendering formats and / or encoding formats. For example, the output from a first rendering node may be point cloud data which logically describes a 3D scene. This point cloud data can be encoded using the techniques of EP21386059.6. A second rendering node may then operate on the point cloud data to generate image data that is more readily displayed by a generic display device, without requiring the display device to model the 3D environment. This image data may be encoded using video coding techniques. The chaining of rendering nodes may be extended to arbitrary tree structures, where a rendering node obtains partially rendered frames from more than one preceding rendering node, and generates further partially or fully rendered frames based on the multiple obtained sequences of partially rendered frames. For example, a content rendering network (CRN) comprising numerous rendering nodes may be used to serve a volumetric event to a large number of same-time users, such as users participating in a shared virtual environment. Rendering the same event for each user is far more expensive in terms of computation time and power consumption than rendering the volumetric effect once and performing the rendering equivalent of multicasting the volumetric effect for multiple users. For example, each user may have a second rendering node (such as a VR headset), and the network may comprise a central first rendering node. The first rendering node may render the volumetric event, and distribute partially rendered frames depicting the volumetric event to the different second rendering nodes. The second rendering node for each user may then integrate the partially rendered frames depicting the volumetric event into a view of the virtual environment which is currently being shown to each user, based on parameters such as the user’s virtual position. The receiver 15, decoder 16 and display device 17 may be consolidated into a single device, or may be separated into two or more devices. For example, some VR headset systems comprise a base unit and a headset unit which communicate with each other. The receiver 15 and decoder 16 may be incorporated into such a base unit. In some embodiments, the network 14 may be omitted. For example, a home display system may comprise a base unit configured as an image source, and a portable display unit comprising the display device 17. In the event that the decoder 16 or the display device 17 does not or cannot handle one or more layers, the receiver 15 or another transmitter associated with the decoder or display device may send a corresponding layer drop indication back through the network 14. The layer drop indication may be received by each rendering node. A rendering node which generates partially or fully rendered frames for that specific decoder or display device may cease generating the dropped layer. On the other hand, a rendering node which generates partially or fully rendered frames for multiple end devices may disregard a layer drop indication received from one end device (as the dropped layer is still needed for other devices). Alternatively, rendering nodes which serve multiple end devices may record received layer drop indications, and may cease generating the dropped layer only when all end devices served by the rendering node indicate that the layer is to be dropped. In preferred examples, the encoders or decoders are part of a tier-based hierarchical coding scheme or format. Hierarchical coding enables frames to be communicated with higher resolution and / or higher frame rate than is possible in single-tier coding schemes. In hierarchical coding, one or more enhancement layers is communicated with base data, where the enhancement layers can be used to up-sample the base data at the decoder, for example providing up-sampling in a spatial or temporal dimension. When combined with equivalent down-sampling of the original frames and generation of the enhancement layer at an encoder, hierarchical coding can overall provide lossless compression of data, with higher resolution and / or higher frame rate for a given transmission bit rate. Examples of a tier-based hierarchical coding scheme include LCEVC: MPEG-5 Part 2 LCEVC (“Low Complexity Enhancement Video Coding”) and VC-6: SMPTE VC-6 ST-2117, the former being described in PCT / GB2020 / 050695, published as WO 2020 / 188273, (and the associated standard document) and the latter being described in PCT / GB2018 / 053552, published as WO 2019 / 111010, (and the associated standard document), all of which are incorporated by reference herein. However, the concepts illustrated herein need not be limited to these specific hierarchical coding schemes. A further example is described in WO2018 / 046940, which is incorporated by reference herein. In this example, a set of residuals are encoded relative to the residuals stored in a temporal buffer. LCEVC (Low-Complexity Enhancement Video Coding) is a standardised coding method set out in standard specification documents including the Text of ISO / IEC 23094-2 Ed 1 Low Complexity Enhancement Video Coding published in November 2021, which is incorporated by reference herein. The system describes above is suitable for generating and presenting a representation of a scene, where this scene displays media content to a user. The scene typically comprises an environment, where the user is able to move (e.g. to move their head or to turn their head) to look around the environment and / or to move around the environment. For example, the scene may be a scene of a room in a building, where the user is able to move around the room (e.g. by moving in the real-world and / or by providing an input to a user interface) in order to inspect various parts ofthe room. Typically, the scene is a XR (e.g. a VR) scene, where the user is able to move about the scene in three degrees of freedom (3DoF) or six degrees of freedom (6DoF) so as to experience the scene. As has been described with reference to Figure 1, the image generator 11 may be arranged to determine point cloud data, where each point of the point cloud has a 3D position and one or more attributes. More generally, the image generator (or another component) is arranged to determine a three-dimensional representation of a scene, where this three-dimensional representation is thereafter used to generate two-dimensional images that are presented to a user at the display device 17. While the points are typically points of a point cloud, more generally the disclosure extends to any point that is associated with a location and a value. Therefore, the points may, more generally, be considered to be data (or datapoints), which data is associated with a location and a value, and the 'points’ may comprise polygons, planes (regular or irregular), Gaussian splats, etc. Referring to Figure 3, there is described a method of determining (an attribute for) a point of such a three-dimensional representation. The method comprises determining the attribute using a capture device, such as a camera or a scanner. The scene may comprise a real scene, in which attribute values are captured using a camera, ora virtual scene (e.g. a three-dimensional model of a scene), in which attribute values are captured using a virtual scanner. Where this disclosure describes ‘determining a point’ it will be understood that this generally refers to determining a point that has a location and an attribute value, where determining the point comprises determining the attribute value and / or storing a point that comprises at least an attribute value and a location value (these values may be indirect values, e.g. where the location is identified relative to another point). Once a plurality of points have been captured, these points can be stored as a three-dimensional representation (e.g. a point cloud) so as to enable the reconstruction of the three-dimensional scene based no this representation. Typically, the scene comprises a simulated scene that exists only on a computer. Such a scene may, for example, be generated using software such as the Maya software produced by Autodesk®. The attributes determined using the methods described herein may then depend on virtual objects located within the scene as well as a virtual lighting arrangement used in the scene. In a first step 11, a computer device initiates a capture process for a capture device, the capture process being initiated with an initial azimuth angle (e.g. of 0°) and an initial elevation angle (e.g. of 0°). In a second step 12, the computer device causes a point to be captured using the capture device at the current azimuth angle and current elevation angle. Capturing a point typically comprises assigning an attribute value to the point, which attribute value may, for example, be a colour of the point and / or a transparency value of the point. Typically, the point has one or more colour values associated with each of a left eye and a right eye of a viewer. Capturing the point may also comprise determining a normal value associated with the point, e.g. a normal of a surface on which the point lies. Typically, capturing the point further comprises determining a location of the point, e.g. by determining a distance of the point from the camera. In practice, determining the point may comprise sending a 'ray' from the capture device and then stepping through a computer model to determine which surface of the computer model is impacted by the ray. The colour, transparency, and normal of this surface are then recorded alongside the distance of the surface from the capture device. In a third step, 13, the computer device determines whether a point has been captured for the capture device at each azimuth of a range of azimuths and in a fourth step 14, if points have not been captured at each azimuth, then the azimuth angle is incremented and the method returns to the second step 12 and another point is captured. The azimuth angle may, for example, be incremented by between 0.01° and 1° and / or by between 0.025° and 0.1°. Typically, the range of azimuth angles is selected to be 360° (i.e. so that the capture device captures points surrounding the entirety of the capture device), but it will be appreciated that other ranges are possible. Once a point has been captured for each azimuth, in a fifth step 15, the computer device determines whether a point has been captured for the capture device at each elevation of a range of elevations and in a sixth step 16, if points have not been captured at each elevation, then the azimuth angle is reset to the initial value, elevation angle is incremented and the method returns to the second step 12 and another point is captured. The elevation angles may, for example, be incremented by between 0.01° and 1° and / or by between 0.025° and 0.1°. Typically, the range of elevation angles is selected to be 360° (i.e. so that the capture device captures points surrounding the entirety of the capture device), but it will be appreciated that other ranges are possible. In a seventh step 17, once points have been captured for each azimuth angle and each elevation angle, the scanning process ends. This method enables a capture device to capture points at a range of elevation and azimuth angles. This point data is typically stored in a matrix. The point data may then be used to provide a representation of the scene to a user, e.g. the three-dimensional representation formed by the point data may be processed to produce two-dimensional images for each eye of a user, with these images then being shown to a user via the display device 17 to provide a virtual reality experience to the viewer. By using the captured data, a video can be provided to a viewer that enables the viewer to move their head to look around the scene (while remaining at the location of the capture device). It will be appreciated that the capture pattern (or scanning pattern) described with reference to Figure 3 is purely exemplary and that numerous capture patterns are possible. In general, the capture process for each capture device comprises capturing one or more points at one or more azimuth angles and / or one or more elevation angles. The ‘points’ captured by the capture device are typically associated with a size, such as a height, a width, or a depth. That is, the points typically relate to two-dimensional planes / pixels and / or three-dimensional voxels. In this regard, there is necessarily some space between the locations of adjacent points (since if the points had no width, then an infinite number of points would be required to capture points at each angle). The size provides points that depict a non-negligible area of the three-dimensional space so that a plurality of points can be fit together to provide a depiction of the scene to a viewer. The width and height of each point is typically dependent on the distance of that point from the capture device, where more distant points have a larger width / height. The width and height of each point is typically determined so that when each point is displayed, there is no space between adjacent points (indeed, there may be some overlap between points to ensure that no gaps appear between points). This height / width of each point can be determined at the time of capturing the points, or can be determined or defined after the capture of the points. Typically, the points comprise a size value, which is stored as a part of the point data. For example, the points may be stored with a width value and / or a height value. Typically, the minimum width and the minimum height of a point are set by the angle increment of the azimuth angle and the elevation angle respectively. The size may bethen specified in terms of this angle increment and / or in terms of this minimum width / minimum height (e.g. as being a multiple of the angle increment). In some embodiments, the size value is stored as an index, which index relates to a known list of sizes (e.g. if the size may be any of 1x1, 2x1, 1x2, 2x2, pixels this may be specified by using 3 bits and a list that relates each combination of bits to a size). The size may be stored based on an underscan value. In this regard, where an object is very near to the viewing zone it may be captured using an unnecessarily dense arrangement of points. Therefore, certain surfaces or areas of the representation may be associated with an underscan value, which underscan value defines a reduction in the number of points captured as compared to a representation without underscan. The size of the points may be defined so as to indicate this underscan value. In an exemplary embodiment, the underscan value is an integer value between 0 and 3 and the size is stored as a combination of point dimensions (e.g. a width in the range [0,2]) and a height in the range ([0,2]) and an underscan factor (e.g. an underscan factor in the range [0,3]). In some embodiments, the width and the height are dependent on the underscan factor. For example, when the underscan factor exceeds a threshold value, the possible height and width values may be limited. In a specific example, when the underscan factor is 3, the width and the height may be limited to the range [0,1]. The size may then be defined as size = underscan*9 + height*3 + width. Such a method provides efficient storage and indication of width, height, and underscan values. As shown in Figure 4a, typically, for each capture step (e.g. each azimuth angle and / or each elevation angle), a plurality of sub-points SP1, SP2, SP3, SP4, SP5 is determined. For example, where the azimuth angle increment is 0.1° then for an azimuth angle of 0°, sub-points may be determined at azimuth angles of-0.05°, -0.025°, 0, 0.025°, and 0.05° (and similar sub-points may be determined for a plurality of elevation angles). Attribute values of these sub-points may then be combined to obtain an attribute value for the point. For example, a maximum attribute value of the sub-points may be used as the value forthe point, an average attribute value of the sub-points may be used as the value forthe point, and / or a weighted average of the sub-points may be used as the value forthe point. It will be appreciated that numerous other methods for combining the attribute values of the sub-points are possible. By determining the attribute of a point based on the attributes of sub-points, the accuracy of the capture process can be increased. While it would be possible to simply reduce the increment of the angle steps to provide a higher resolution scene, by considering sub-points but only storing attributes for points, a balance can be struck between accuracy and file size (since storing every sub-point would lead to a substantial increase in the amount of data that needs storing). With the example of Figure 4a, for each point of the three-dimensional representation that is captured by a capture device, this capture device may obtain attributes associated with each of the sub-points SP1, SP2, SP3, SP4, SP5, combine these attributes to obtain a point attribute, and then store a point with a distance that is an average (e.g. a weighted average) of the distances of the sub-points from the capture device, at the nominal angle of the point, with the point attribute. As shown in Figure 4b, where a plurality of sub-points SP1, SP2, SP3, SP4, SP5 are considered, these points may have different distances from the location of the capture device. In some embodiments, the attributes of the sub-points may be combined in dependence on this distance, e.g. so that sub-points nearer to the capture device have higher weightings. However, the possibility of sub-points with substantially different distances raises a potential problem. Typically, in order to determine a distance for a point, the distances forthe sub-points are averaged. But where the sub-points have substantially different distances and / or are related to different surfaces in the scene, this may result in the point having a distance that does not correspond to any actual surface in the scene. Therefore, the point may seem to hang in space (e.g. to hang between the front and rear surfaces shown in Figure 4b. Similarly, where the attribute values of the sub-points greatly differ, e.g. if the sub-points SP1 and SP2 are white in colour and the sub-points SP3 and SP4 are black in colour, then the attribute value of the point may be substantially different to the attribute value of other points in the scene. In an example, if the scene were composed of black and white objects, the point may appear as a grey point hanging in space between these objects. In some embodiments, the computer device is arranged to aggregate sub-points so as not to create any floating points. For example, the computer device may determine whether the sub-points are spatially coherent by employing a clustering algorithm (e.g. a k-means clustering algorithm). Where the sub-points are spatially coherent (e.g. where a difference in the distance of the sub-points is below a threshold value), these distances may be averaged to obtain a distance for the point. Where the sub-points are not spatially coherent, the sub-points may be processed to ensure that the distance of any point places it upon a surface; for example, in the system of Figure 4b, sub-points SP1, SP2, and SP3 may be grouped into a first point and sub-points SP4 and SP5 may be grouped into a second point. Since each sub-point is associated with the same capture device and capture angle (all of these sub-points being associated with a capture step that has a particular azimuth angle and elevation angle), these points may be located at the same angle with respect to a capture device. Therefore, to ensure that each sub-point affects the representation considered, the first point (made up of sub-points SP1, SP2, and SP3) may have a smaller distance value than the second point (made up of sub-points SP4 and SP5) and the first point may be assigned a nonzero transparency value so that the second point can be seen through the first point. By capturing points at a plurality of azimuth angles and elevation angles, e.g. using the method described with reference to Figure 3, it is possible to provide a three-dimensional representation of the scene that can later be used to enable a viewer to view the scene from a plurality of angles. More specifically, given the three-dimensional points captured by the capture device, a computer device is able to render a two-dimensional representation (e.g. a two-dimensional image) of the scene for each eye of a viewer so as to provide a representation with an impression of depth. The computer device may render a series of two-dimensional representations to enable the viewer to look around the scene, where the two-dimensional representations are rendered based on an orientation of the viewer’s head. In this way, the determined representation is useable to provide, for example, a virtual reality (VR), mixed reality (MR), augmented reality (AR), and / or extended reality (XR) experience to the viewer. To enable such a display, the display device 17 is typically a virtual reality headset, that comprises a plurality of sensors to track a head movement of the user. By tracking this head movement, the display device is able to update the images being displayed to the viewer as the viewer moves their head to look about the scene. Typically, this involves the display device sensing the sensor data to an external computer device (e.g. a computer connected to the display device via a wire). The external computer device may comprise powerful graphical processing units (GPUs) and / or computer processing units (CPUs) so that the external computer device is able to rapidly render appropriate two-dimensional images for the viewer based on the three-dimensional images and the sensor data. In some embodiments, the external computer device may comprise a server device, where the display device 17 may be connected to this server device wirelessly. This enables the two-dimensional images to be streamed from the serverto the display device so as to enable the display of high-quality images without the need for a viewer to purchase expensive computer equipment. In other words, operations that require large amounts of computing power, such as the rendering of two-dimensional images based on the three-dimensional representation, may be performed by the server, so that the display device is only required to perform relatively simple operations. This enables the experience to be provided to a wide range of viewers. In some embodiments, a first two-dimensional image is provided to the display device 17 (and / or a connected device) and this first image is ‘warped’ in order to provide an image for viewing at the display device. The warping of the image comprises processing the image based on the sensor data in order to provide an image that matches a current viewpoint of the viewer. By performing the warping at the display device or another local device, the lag between a head movement of the user and an updating of the two-dimensional representation of the scene can be reduced. One issue with the above-described method of capturing a three-dimensional representation is that it only enables a viewer to make rotational movements. That is, since the points are captured using a single capture device at a single capture location, there is no possibility of enabling translational movements of a viewer through a scene. This inability to move translationally can induce motion sickness within a viewer, can reduce a degree of immersion of the viewer, and can reduce the viewer’s enjoyment of the scene. Therefore, it is desirable to enable translational movements through the scene. To enable such movements, the three-dimensional representation of the scene may be captured using a plurality of capture devices placed at different locations (or the same capture device placed at different locations). A viewer is then able to move around the scene translationally (e.g. by moving between these locations). More generally, by capturing points for every possible surface that might be viewed by a viewer, a three-dimensional representation of a scene may be captured that allows a suitable two-dimensional representation ofthis scene to be rendered regardless of a location of a viewer (e.g. regardless of where a user is standing within a virtual room). This need to capture points for every possible surface (so as to enable movement about a scene) greatly increases the amount of data that needs to be stored to form the three-dimensional representation. Therefore, as has been described in the application WO 2016 / 061640 A1, which is hereby incorporated by reference, the three-dimensional representation may be associated with a viewing zone, or a zone of viewpoints (ZVP), where the three-dimensional representation is arranged to enable a user to move about the viewing zone so as to view the scene. Figure 5 illustrates such a viewing zone 1 and illustrates how the use of a viewing zone limits the amount of image data that needs to be stored to provide a three-dimensional representation of the scene. With the scene shown in this figure, and the viewing zone 1 shown in this figure, it is not necessary to determine attribute data for the occluded surface 2 since this occluded surface cannot be viewed from any point in the viewing zone. Therefore, by enabling the userto only move within the viewing zone (as opposed to around the whole scene) the amount of data needed to depict the scene is greatly reduced. While Figure 5 shows a two-dimensional viewing zone, it will be appreciated that in practice the viewing zone 1 is typically a three-dimensional zone or volume. The viewing zone 1 may, for example, comprise a rectangular volume, or a rectangular parallelepiped, and the viewing zone may have a height of at least 30 cm, a depth of at least 30 cm, and / or a width of at least 30 cm, where these dimensions enable a user to move their head while remaining in the viewing zone. This is merely an exemplary arrangement of the viewing zone; it will be appreciated that viewing zones of various shapes and sizes may be used (e.g. spherical viewing zones). That being said, it is preferable that the viewing zone is limited so as to cover only a part of the volume of the scene, e.g. no more than 50% of the scene no more than 25% of the scene, and / or no more than 10% of the scene. In this regard, if the viewing zone is the same size as the scene, then the three-dimensional representation will simply be a standard representation for virtual reality (that enables a user to move freely about the scene) - and so the use of the viewing zone will not provide any reduction in file size. The viewing zone 1 enables movement of a viewer around (a portion of) the scene. For example, where the scene is a room, the base representation may enable a userto walk around the room so as to view the room from different angles. In particular, the viewing zone enables a user to move through the scene with six degrees-of-freedom (6DoF) movement through the scene, where this aids in the provision of an immersive experience. In some embodiments, the viewing zone 1 may be four-dimensional, where a three-dimensional location of the viewing zone changes over time - and in such embodiments the size and location of the occluded surface 2 may also change over time. More generally, it will be appreciated that viewing zones may be formed in any size or shape, with different sizes and shapes being suitable for different scenes. The volume of the viewing zone 1 is typically selected so that a user is able to move to a degree sufficient to avoid motion sickness and to provide an immersive sensation, while still only enabling a limited amount of movement (where this leads to a smaller file size as compared to an implementation where a user is able to fully move about the scene). Typically, the viewing zone is arranged to enable a user to move their head while they are sitting or standing, but not to freely roam around a room. The viewing zone 1 may have a (e.g. real-world) volume of less than five cubic metres (5m3), less than one cubic metre (1m3), less than one-tenth of a cubic metre (0.1m3) and / or less than one-hundredth of a cubic metre (0.01m3). The viewing zone 1 may also have a minimum size, e.g. the viewing zone may have a volume of at least 1% of the volume of the scene, at least 5% of the volume of the scene, and / or at least than 10% of the volume of the scene. Similarly, the viewing zone may have a volume of at least one-thousandth of a cubic metre (0.01m3); at least one-hundredth of a cubic metre (0.01m3); and / or at least one cubic metre (1m3). The 'size' of the viewing zone 1 typically relates to a size in the real world, where if the viewing zone has a length of one metre this means that a user is able to move one metre in the real world while staying within the viewing zone. The size of the viewing zone in the scene may be greater than, equal to, or less than the size of the viewing zone in the real world. For example, the viewing zone may scale a real-world distance so that moving one metre in the real world moves the user less than (or more than) one metre in the scene. This enables the scene to provide different perceptions to the user (e.g. to make the user feel larger or smaller than they are in real life). Similarly, the viewing zone may scale a real-world angle so that rotating one degree in the real world rotates the user less than (or more than) one degree in the scene. Therefore, a viewing zone with a volume of one cubic metre typically connotes a viewing zone in which the user is able to move about a one cubic metre volume in the real world while remaining in the viewing zone. And this may cause the user to move about a volume that is more than, or less than, one metre in the scene. Referring to Figure 6a, in order to capture points for each surface and location that is visible from the viewing zone 1, a plurality of capture devices C1, C2, ..., C9 may be used (e.g. a plurality of virtual scanners and / or a plurality of cameras). Each capture device is typically arranged to perform a capture process, e.g. as described with reference to Figure 3, in which the capture device captures points at a plurality of azimuth angles and elevation angles. By locating the capture devices appropriately, e.g. by locating a capture device at each corner of the viewing zone, it can be ensured that most (or all) points of a scene are captured. Typically, a first capture device C1 is located at a centrepoint of the viewing zone 1. In various embodiments, one or more capture devices C2, C3, C4, C5 may be located at the centre of faces of the viewing zone; and / or one or more capture devices C6, C7, C8, C9 may be located at edges of and / or corners of the viewing zone. Figure 6a shows a two-dimensional view (e.g. a plan view) of a rectangular viewing zone. It will be appreciated that within this viewing zone each capture device may be located on a shared plane. Equally, the various capture devices may be located on different planes. Referring, for example, to Figure 6b, there is shown a three-dimensional view of a cuboid viewing zone, where there is a capture device located: at the centre of the viewing zone; at the centre of each face of the viewing zone; and at each corner of the viewing zone. With this arrangement, many locations in the scene (e.g. specific surfaces) will be captured by a plurality of capture devices so that there will be overlapping points relating to different capture devices. This is shown in Figure 7, which shows a first point P1 being captured by each of a first capture device C1, a sixth capture device C6, and a seventh capture device C7. Each capture device captures this point at a different angle and distance and may be considered to capture a different 'version' of the point. Typically, only a single version of the point is stored, where this version may be the highest quality version of the point and / or may be the version of the point associated with the nearest and / or least angled capture device. The highest ‘quality’ version of the point is typically captured by the capture device with the smallest distance and smallest angle to the point (e.g. the smallest solid angle). In this regard, as described with reference to Figures 4a and 4b, capturing a point for a given azimuth angle and elevation angle typically comprises capturing a plurality of sub-points at varying sub-point azimuth and elevation angles spread around the point azimuth and elevation angles. Due to the different spreads of sub-points, each capture device will capture a different version of the point (that has a different attribute) even when the points are at the same location. Capture devices that are close to the point and less angled with respect to the point typically have a smaller spread of sub-points and so typically obtain a version of a point that is sharper than a version of that point captured by more distant capture devices. In some embodiments, a quality value of a version of the point is determined based on the spread of subpoints associated with this version (e.g. based on the perimeter formed by these sub-points and / or based on a surface area or volume bounded by these sub-points). The version of the point that is stored may depend on the respective quality values of possible versions of the points. Regarding the ‘versions’ of the points, it will be appreciated that two 'points’ in approximately the same location captured by each capture device may not have exactly the same location in the three-dimensional representation. More specifically, since each capture device typically projects a ’ray’ at a given angle, the rays of differing capture devices may contact the surface at different locations for each capture device. Two points may be considered to be two ‘versions’ of a single point when they are within a certain proximity, e.g. a threshold proximity. For example, where the first capture device C1 captures a first point and a second point at subsequent azimuth angles, and the sixth capture device C6 captures a further point that is in between the locations of the first point and the second point, this further point may be considered to be a ‘version’ of one of the first point and the second point. This difference in the points captured by different capture devices is illustrated by Figures 8a and 8b, which show the separate captured grids that are formed by two different capture devices. As shown by these figures, each capture device will capture a slightly different ‘version’ of a point at a given location and these captured points will have different sizes. Each capture step is associated with a particular range of angles (e.g. a nominal capture angle of 1° might encompass angles from 0.9° to 1.1°), and therefore capture devices that are far from a point to be captured represent a wider region at the capture distance than capture devices closer to that point to be captured. As shown in Figure 8a, the capture device C1 would capture the points P1 and P2 in separate brackets, whereas for the capture device C2 these points are in the same bracket. Therefore, the capture device C2 might determine a single point that encompasses both points P1 and P2, whereas the capture device C1 would determine separate points for these two points. Considering then a situation in which points P1 and P2 are captured separately, and capture device C1 is used to capture point P1 while capture device C2 being used to capture point P2, it should be apparent that the 'sizes’ of these captured points, and the locations in space that are encompassed by the captured points will be based on different grids. For example, the width of the captured point P2 captured by the capture device C2 will be larger than the width of the captured point P1 captured by the capture device C1. The capture process may be determined based on the existence of these different grids, and on the different bracket widths that occur at different distances from a capture device. Figure 8a shows an exaggerated difference between grids for the sake of illustration. Figure 8b shows a more realistic embodiment in which the three-dimensional representation comprises a plurality of points associated with different capture devices, where these points lie on different grids associated with these different capture devices. In order to store the points of the three-dimensional representation, the points may be stored as a string of bits, where a first portion of the string indicates a location of the point (e.g. using x, y, z coordinates) and a second portion of the string locates an attribute of the point. In various embodiments, further portions of the string may be used to indicate, for example, a transparency of the point, a size of the point, and / or a shape of the point. A computer device that processes the three-dimensional representation after the generation of this representation is then able to determine the location and attribute of each point so as to recreate the scene. This location and attribute may then be used to render a two-dimensional representation of the scene that can be displayed to a viewer wearing the display device 17. Specifically, the locations and attributes of the points of the three-dimensional representation can be used to render a two-dimensional image for each of the left eye of the viewer and the right eye of the viewer so as to provide an immersive extended reality (XR) experience to the viewer. The present disclosure considers an efficient method of storing the locations of the points (e.g. at an encoder) and of determining the locations of the points (e.g. at a decoder). As has been described with reference to Figures 5a and 5b, the points of the three-dimensional representation are determined using a set of capture devices placed at locations about the viewing zone, where these capture devices are arranged to capture points at a series of azimuth angles and elevation angles. Typically, each of the capture devices is arranged to use the same capture process (e.g. the same series of azimuth angles and elevation angles), though it will be appreciated that different series of capture angles are possible. For example, there may be a plurality of possible series of capture angles, where different capture devices use different capture angles. In general, the present disclosure considers a method in which points are stored based on a capture device identifier and an indication of a distance of the point from the capture device associated with this capture device identifier. Typically, the point is also associated with an angular indicator, which indicates an azimuth angle and / or an elevation angle of the point relative to the identified capture device. It will be appreciated that the storage of the distance and the angle may take many forms. For example, the distance and the angle of each point may be converted into a universal coordinate system, where each capture device has a different location in this universal coordinate system. In particular, each point may be stored with reference to a centre of this universal coordinate system, which centre may be co-located with a central capture device. Where a point is determined based on a distance and an angle from a capture device of a known location in this universal coordinate system, the coordinates of the point in this universal coordinate system can be determined trivially - and the location of the point may then be stored either relative to the capture device or as a coordinate in the universal coordinate system. The capture device identifier may comprise a location of a capture device (e.g. a location in a co-ordinate system of the three-dimensional representation). Equally, the capture device identifier may comprise an index of a capture device. Similarly, the indication of the azimuth angle and the elevation angle for a point may comprise an angle with reference to a zero-angle of a co-ordinate system of the three-dimensional representation. Equally, the azimuth angle and / or the elevation angle may be indicated using an angle index. In some embodiments, the three-dimensional representation is associated with configuration information, which configuration information comprises one or more of: a set of capture device indexes; locations associated with the capture devices and / or the capture device indexes; a spacing of capture devices (e.g. so that locations of the capture devices can be determined from a location of a first capture device and the spacing); angles associated with a capture process for the capture devices; an azimuth angle increment and / or an elevation angle increment associated with the capture process; and a set of angle indexes (e.g. to match an angle index to an angle). With this configuration information, it is possible to determine a location of each capture device from an index of that capture device and / orto determine a capture angle from a known capture process. Therefore, given two numbers: a capture device index and an angle index (that is associated with a combination of a specific azimuth angle and a specific elevation angle), a location of a capture device and a direction of a point from this capture device can be determined. By also signalling a distance of the point from the signalled capture device, a precise location of the point in the three-dimensional space can be signalled efficiently. Typically, the point is associated with each of: a camera index, a distance, an first angular index (e.g. a first azimuth), and a second angle (e.g. a second elevation) This method of indicating a location of a point enables point locations to be identified using a much smaller number of bits than if each point location is identified using x, y, z coordinates. Referring to Figure 9, there is shown a method of determining a location of a point. This method is carried out by a computer device, e.g. the image generator 11 and / or the decoder 15. In a first step 21, the computer device identifies an indicator of a capture device used to capture the point. Typically, this comprises identifying a portion of a string of bits associated with a capture device index. In a second step 22, the computer device identifies an indicator of an angle of the point from the capture device. Typically, this comprises identifying an angle index, e.g. an azimuth index and / or an elevation index and / or a combined azimuth / elevation index, which index(es) identifies a step of the capture process during which the point was captured. In a third step 23, based on the identifiers, the computer device determines the location of the capture device and the angle of the point from the capture device. The capture device identifier is typically a capture device index, which is related to a capture device location based on configuration information that has been sent before, or along with, the point data. For example, the configuration information may specify: Location of first capture device is (0,0,0). Step between capture devices is (0,0,1) along the grid, then across the grid, then up the grid. - The grid is (10,10,10). With this information, a capture device with an index of 1 can be determined to be located at (0,0,0); a capture device with an index of 5 can be determined to be located at (0,0,4); a capture device with an index of 12 can be determined to be located at (0,1,0), and so on. Equally, the configuration information may specify a list of camera indexes and locations associated with these indexes, where this enables the use of a wide range of setups of capture devices. Typically, the three-dimensional representation is associated with a frame of video. The configuration information may be constant over the frames of the video so that the configuration information needs to be signalled only once for an entire video. Therefore, the configuration information may be transmitted alongside a three-dimensional representation of a first frame of the video, with this same information being used for any subsequent frames (e.g. until updated configuration information is sent). The angle identifier may similarly be related to an angle by a location and an increment that are signalled in a configuration file. For example, the configuration information may specify: An azimuth increment and an elevation increment are each 1°. There are 359 increments for each angle type. With this information: a capture angle with an index of 1 can be determined to be at an azimuth angle of 0° and an elevation angle of 0°; a capture angle with an index of 10 can be determined to be at an azimuth angle of 10° and an elevation angle of 0°; a capture angle with an index of 360 can be determined to be at an azimuth angle of 0° and an elevation angle of 1°; and a capture angle with an index of 370 can be determined to be at an azimuth angle of 9° and an elevation angle of 1°; etc. In a fourth step 24, based on the determined location of the capture device and the determined angle, a location of the point is determined. Typically, this comprises determining the location of the point based on the location of the capture device, the capture angle, and a distance of the point from the capture device (where this distance is specified in the point data for the point). Determining the location of the point typically comprises determining the location of the point relative to a centrepoint of the three-dimensional representation, This location of the point may then be converted into a desired coordinate system and / orthe point may be processed based on its location (e.g. to stitch together adjacent points). The angular identifier typically comprises a first angular identifier and a second angular identifier, where the first identifier provides the azimuthal angle of the point and the second identifier provides the elevation angle of the point. Referring to Figure 9, each angular identifier may be provided as an index of a segment of the three-dimensional representation, where, for example, an index of 0 may identify the point as being in a first angular bracket 101 and an index of 1 may identity the point as being in a second angular bracket 102. In this regard, the capture devices are arranged to perform a capture process, e.g. as described with reference to Figure 3, with a non-infinite angular resolution. Given this non-infinite resolution, each point is not a one-dimensional point located at a precise angle. Instead, each point is a point for a particular area of space, with the size of this area being dependent on the angular resolution as well as the distance of the point from the capture device. In other words, each capture angle determines a point for an angular range (with the range being dependent on the angular resolution). That is, if the capture process leads to points being captured at angles of 10°, 11°, and 12° then this can equally be considered to relate to points being captured at a first range of 9.5°-10.5°, a second range of 10.5°-11.5°, and a third range of 11.5°-12.5°. This is shown in Figure 9, which shows a series of angular brackets, with the size of these angular brackets at a given distance being dependent on the angular resolution. The angular identifier(s) typically comprise a reference to such an angular bracket. Consider, for example, a cube placed with the capture device C1 at the centre of this cube. By dividing this cube into x segments at regular azimuth angles and y segments at regular elevation angles, it is possible to identity any angular range of the representation by reference to an x segment and a y segment (and then the space bracketed by this angular range will depend on both the angular resolution (e.g. the angle between adjacent brackets) and the distance of the point from the capture device). Typically, each capture device has the same capture pattern so that the angular bracketing of each device is the same (albeit centred differently at the location of the relevant capture device). For example, in an embodiment with 1000 equal angular brackets, the angle for each bracket may be 360 / 1000. In some embodiments, different capture devices are associated with different capture patterns, where this may be signalled in configuration information relating to the three-dimensional representation. In some embodiments, each capture device is arranged to capture a point for a plurality of angular brackets, where each bracket is associated with a different angle. The angular spread of each bracket (that is, the angle between a first, e.g. left, angular boundary of the bracket and a second, e.g. right, angular boundary of the bracket) may be the same; equally, this angular spread may vary. In particular, the angular spread may vary so as to be smaller for points which are directly in front of (or behind, or to a side of) the capture device. For example, the embodiment shown in Figure 10 shows an angular bracketing system that is based on a cube. With this system, a cube is placed such that a capture device is located at the centre of the cube and the cube is then split into 1000 sections of equal size (it will be appreciated that the use of 1000 sections is exemplary and any number of sections may be used). Each of these sections is then associated with an angular index. With this arrangement, the angular spread of each section (or bracket) varies, as has been described above. Figure 9 shows a two-dimensional square, where each angular bracket of the square is referenced by an index number (between 1 and 100). In a three-dimensional implementation, an angular bracket of a cube could be indicated with two separate numbers (with a first azimuthal indicator that identifies a ‘column’ of the cube and a second elevational indicator that identifies a ‘row’ of the cube). Equally, a singular indicator may be provided that indicates a specific bracket of the cube. Therefore, for a cube that is divided into 1000 elevational sections and 1000 azimuthal sections, the bracket may be indicated with two separate indicators that are each between 0 and 999 or with a single indicator that is between 0 and 999999. It will be appreciated that the use of a cube to define the brackets is exemplary and that other bracketing systems are possible. For example, a spherical bracketing system may be used (where this leads to curve angular brackets). Equally, a lookup table may be provided that relates angular indexes to angles, where this enables irregularly spaced brackets to be used. Typically, determining the location of the point comprises determining the location of the point so as to be at the centre of the angular bracket identified by the angular identifier(s). Video background The display device 17 is arranged to display one or more two-dimensional images (hereafter termed one or more two-dimensional ‘immersive’ images) to a user in order to provide the impression of a three-dimensional scene. In particular, the display device may provide a first ‘immersive’ image for a first eye of a user and a second (different) ‘immersive’ image for a second eye of a user with the differences between the images providing the impression of depth to a viewer of the images (‘immersive’ is used here as a label to distinguish the aforementioned displayed two-dimensional images from other two-dimensional images). The immersive images are typically arranged to be viewed by the display device 17 (e.g. by a VR headset). Typically, a computer device (e.g. the image generator 11 or the display device 17) is arranged to form the two-dimensional images based on a three-dimensional representation of the scene (e.g. based on a point cloud). Typically, the three-dimensional representation comprises a plurality of points, where each point has a location, an attribute value for a left eye, and an attribute value for a right eye. Based on a position of a viewer (e.g. in the viewing zone), the computer device is able to identify the points of the three-dimensional representation that are visible to the user and to form the two-dimensional immersive images for each eye based on the attribute values of these points and the locations of these points relative to the viewer. Forming the two-dimensional immersive images in this way enables accurate images to be formed that enable the provision of an immersive scene to a user; however, negatively, this tends to require large amounts of computing power to identify, evaluate, and convert each of the relevant points in the representation. Therefore, it is desirable to identify methods that reduce the amount of computation necessary to form the two-dimensional immersive images from the three-dimensional representation. Referring to Figure 10, there is described a method of forming a two-dimensional immersive image based on the three-dimensional representation and a (different) two-dimensional image. This method may be carried out by a computer device (e.g. by the image generator 11 or the display device 17). The different two-dimensional image is described below as a ‘background’ or‘first’ two-dimensional image, but it will be appreciated that this ‘background’ image does not need to form a background of the scene. The terms ‘background’ and ‘immersive’ are not intended to be limiting terms and are instead used to distinguish between a first two-dimensional image that is included in a three-dimensional representation and a second two-dimensional image that is formed from the three-dimensional representation (and the first two-dimensional image). In a first step 11, the computer device identifies a three-dimensional representation of a scene. In a second step 12, the computer device identifies a first, e.g. background, two-dimensional image (and / or video) associated with the scene. In a third step 13, the computer device determines a second, e.g. immersive, two-dimensional image based on the three-dimensional representation and the two-dimensional image. Typically, the first two-dimensional image comprises a background image, where two-dimensional objects formed based on the three-dimensional representation are then superimposed onto this background image. The method may comprise identifying a plurality of first two-dimensional images and then superimposing these images onto each other and / or positioning these images relative to each other so as to form a layered two-dimensional immersive image. Such an image may provide a parallax effect between the layered two-dimensional images in the two-dimensional immersive image. Using a first two-dimensional image (or video) to form a background of the two-dimensional immersive image enables a substantial reduction in the computing power required to generate the two-dimensional immersive image. In this regard, instead of determining location and attribute information for a plurality of different points in order to generate a scene background, the computer device is able to identify a single two-dimensional image to form this background. The two-dimensional background image is typically stored separately to the three-dimensional representation, where this two-dimensional background image may then be encoded / decoded using different techniques and / or different hardware. The two-dimensional background image typically comprises a monoscopic image (where the same image is used for each of the left eye and the right eye of a viewer). Equally, the background image may comprise a stereoscopic image (or a plurality of stereoscopic images) formed of a plurality of component images (where different component images are used for the left eye and the right eye of a viewer). In some embodiments, monoscopic background images are used for points that are above a threshold distance from the viewing zone with stereoscopic background images being used for points below this threshold distance. In some embodiments, monoscopic background images are used to replace points that are above a first threshold distance from the viewing zone with stereoscopic background images being used to replace points below this first threshold distance and above a second threshold distance and points being unmodified if they are below the second threshold distance from the viewing zone. The background two-dimensional image may be selected at the time of generating (e.g. rendering) the two-dimensional immersive image. For example, the background two-dimensional image may be selected by a user. The two-dimensional background image may be selected from a plurality of possible images; for example, the two-dimensional background image may be selected so as to show one of: a night background, a daytime background, a sky background from the perspective of Earth, a sky background from the perspective of the moon, etc.. Therefore, the two-dimensional background image may be selected so as to provide a plurality of different two-dimensional immersive images, where this two-dimensional immersive image can be personalised for a specific user. For example, the same foreground scene may be superimposed onto a variety of background scenes based on (e.g. the context of) a viewer of the scene. Typically, the background two-dimensional image is quicker to provide and edit than the three-dimensional representation, so that the background two-dimensional image can be more readily updated so as to edit the two-dimensional immersive image. Such a two-dimensional background image is particularly beneficial for providing a distant background for the scene. In this regard, viewers of the scene will be better able to distinguish between proximal surfaces when these surfaces are close to the viewer (e.g. a separation of 5 cm in depth at a distance of 1 km is typically indistinguishable to a viewer while a separation of 5 cm at a distance of 50 cm will typically be highly perceived). Therefore, it is possible to replace a plurality of points at such a distance with an image (that may comprise a smaller amount of information, e.g. depth information) without substantially reducing the quality of an immersive image formed based on a representation. In particular, at large distances viewers are not capable of identifying parallax effects that occur as objects move relative to each other and viewers are also not able to distinguish between stereoscopic images and monoscopic images. Therefore, at these large distances complex points can be replaced by simple monoscopic images. In some embodiments, the method comprises identifying one or more points of the three-dimensional location that are more than a threshold distance from a viewer of the scene (at a given point in time) and / or that are more than a threshold distance from a viewing zone of the three-dimensional representation. These points may then be designated as ‘background points’, where these points may be associated with two-dimensional background images and / or these points may be ignored when forming the two-dimensional immersive image, with an identified two-dimensional image being used instead for the relevant area of the scene. The threshold distance may, for example, be 500 metres or 1000 metres. The threshold distance may be dependent on the representation or on an expected rendering of the representation. For example, the threshold distance may be dependent on a resolution of the representation or a complexity of the representation. Equally, the threshold distance may be defined by a user input. In some embodiments, the computer device is arranged to identify background points of a representation and to thereby identity remaining (e.g. ‘foreground’) points of the representation. These foreground points may be used to form a new three-dimensional representation (e.g. by deleting the background points), where this new three-dimensional representation can then be readily combined with a two-dimensional image in order to form the two-dimensional immersive image without re-considering whether or not to render individual points of the new three-dimensional representation. In other words, the new three-dimensional representation will contain gaps (where the background points were originally located) so that this new three-dimensional representation can readily be placed onto different backgrounds. In some embodiments, the method comprises replacing one or more points of a three-dimensional representation with a two-dimensional image (or a reference to a two-dimensional image). This two-dimensional image may then be treated as a (new) point or element in the representation so that a user is able to modify the representation by modifying the two-dimensional image (e.g. to change a location or a size of the image). In some embodiments, the method comprises identifying the locations of one or more background points in the three-dimensional representation and combining the three-dimensional representation with the background two-dimensional image (so as to generate the two-dimensional immersive image) based on the locations of the background points. In some embodiments, the two-dimensional background image is not associated with a specific location but rather is located behind the three-dimensional representation. This may comprise forming an initial two-dimensional immersive image based on the three-dimensional representation and then imposing this initial two-dimensional immersive image onto the background image (so that the background image is visible via any holes in the three-dimensional representation (e.g. holes formed by removing background points from the three-dimensional representation). In these and other embodiments, the two-dimensional background image may comprise a 360° image that is designed to wrap around the representation to provide an entire background for a scene. The first (background) two-dimensional image that is identified in the first step 11 may be selected by a user; for example, a user generating an immersive scene may select a two-dimensional image that is used to generate the two-dimensional immersive image. In some embodiments, the two-dimensional background image is selected, e.g. in part, based on a feature of the three-dimensional representation. For example, the two-dimensional background image may be selected to replace (or to be rendered instead of) one or more points based on a characteristic of these points. In particular, background the two-dimensional image may be identified based on the distances of one or more points from a capture device, a position of a user, or a viewing zone of the scene. For example, a resolution of the image may be selected based on a distance of the points from the viewing zone; in an example, a first point that is a first distance from the viewing zone is associated with a first background image and a second point that is a second distance from the viewing zone is associated with a second background image. Where the first distance is less than the second distance, the first background image and the second background image may be selected such that the resolution of the first background image is greater than the resolution of the second background image. In practice, this selection based on a feature of the three-dimensional representation may involve using different background images depending on the scene being shown: where the scene is outside, the 'background' may be a distant view of space that may be at a low resolution; where the scene is inside, the ‘background’ may be a relatively close wall that may be at a high resolution. In a practical example, the two-dimensional image may be arranged to provide a background that is far from the viewing scene (e.g. the sky), where this background is used to form the two-dimensional immersive image instead of one or more points of the three-dimensional representation. In some embodiments, the computer device is arranged to identify one or more replaceable points in the representation, which replaceable points could be replaced by a two-dimensional image. In some embodiments, these points are removed from the three-dimensional representation and / or are replaced by new points or elements relating to the two-dimensional image. In some embodiments, the computer device marks these replaceable points (so that they can be replaced at a later time and / or not used in the determination of the two-dimensional immersive image). In some embodiments, the computer device determines the two-dimensional immersive image in dependence on the replaceable points (e.g. the computer device may determine this two-dimensional immersive image based on points of the three-dimensional representation that do not include the replaceable points and / or the computer device may determine the two-dimensional immersive image by inserting the identified two-dimensional image into the two-dimensional immersive image at a location of the replaceable points. In some embodiments, the replaceable points are removed from the representation without being replaced. This leaves blank spaces in the two-dimensional immersive image that can then be filled in with a background image by compositing this two-dimensional immersive image with a background image. Referring to Figure 11, there is described a method of modifying a point of a three-dimensional representation based on a feature of that point. This method may be carried out by a computer device (e.g. by the image generator 11 or the display device 17). In a first step 31, the computer device identifies a point in a three-dimensional representation of a scene. In a second step 32, the computer device identifies a characteristic of the point. This characteristic typically comprises a distance of the point from a viewing zone of the scene. In some embodiments, this characteristic comprises an attribute of the point, e.g. a colour of the point. In some embodiments, the characteristic comprises a difference between a plurality of the points; for example, the computer device may identify points for which a difference between an attribute for a left eye and an attribute for a right eye is below a threshold difference. Such points may be replaced with a two-dimensional image that may have only a single value (which single value can then be used for both eyes of the user). In a third step 33, the computer device modifies the point based on a value ofthe characteristic. In particular, the computer device may remove the point from the three-dimensional representation and / or the computer device may replace an attribute value of the point with a (e.g. reference to a) two-dimensional image. The device may replace attribute values of each of a left eye attribute and a right eye attribute with this reference to a two-dimensional image, where this provides additional space for the reference (to enable referencing of a broad range of two-dimensional images). In some embodiments, the characteristic relates to a change of a location and / or an attribute of a point over a plurality of three-dimensional representations. Where the three-dimensional representation is associated with a video, the computer device may be arranged to identify a plurality of three-dimensional representations associated with different frames of the video. The computer device may then be arranged to identify a ‘static’ point within this representation based on a similarity of the location and / or attribute of a point over a plurality of representations. In particular, the computer device may be arranged to identify points with the same location in a plurality of three-dimensional representations and the computer device may then identify these points as a persistent point (e.g. relating to a stationary surface). The computer device may then identify this point as a replacement point. Equally, the computer device may assess attributes (or a change of an attribute) of the points and the computer device may identify the point(s) as a replacement points based on these attributes. In a practical example, a plurality of points that are spatially persistent over a plurality of three-dimensional representations may be replaced with a pointer to an image or a video. This image or video may be stored separately to the three-dimensional representation and then combined with (two-dimensional objects formed based on) the three dimensional representation at the time of rendering the two-dimensional immersive image. Referring to Figure 12, a method of identifying such static points may comprise: in a first step 41, identifying a first point in a first three-dimensional representation; in a second step 42, identifying a second point in a second three-dimensional representation; and in a third step 43, determining that a point is a static point based on the locations and / or attributes of the first point and the second point. In particular, the computer device may identify that a plurality of points in a plurality of three-dimensional representations (relating to successive frames of a video) are spatially related (e.g. where a difference in the distances of the points is below a threshold distance) and / or have related (e.g. similar) attributes. Such points may be identified as static points and / or background points, which points may be suitable for being replaced with a two-dimensional image or video. In some embodiments, the replacement points may be extracted from the three-dimensional representation(s) and used to form a two-dimensional image and / or video and / or to form a further three-dimensional representation. In particular, the replacement points may be extracted from the three-dimensional representation, converted into a two-dimensional image, and this two-dimensional image may be the identified two-dimensional image of the second step 22 of the method of Figure 11. Such a process enables the determined two-dimensional image to be processed and encoded using different techniques than the three-dimensional representation so as to improve the efficiency with which the scene can be provided. In some embodiments, the characteristic relates to a size of the point and / or the size of a plurality of identified points. In particular, the computer device may be arranged to identify a plurality of spatially related points (e.g. a plurality of points on a surface and / or a plurality of points that maintain a consistent spatial relationship throughout a plurality of three-dimensional representations, and the computer device may be arranged to identify these points as replacement points based on a number and / or a size of the points exceeding a threshold number / size. The above description has primarily related to identifying or modifying a point of a representation in order to replace this point with an image. In practice, the above-described methods typically involve identifying a plurality of points that can be replaced with an image. In practice, the 'background' of a scene typically comprises a large part of, or even a majority of, a scene, where a number of active elements are then rendered in front of this background. Typically, the method comprises replacing a large number of points with comparably (large) background that is an image. This may comprise replacing an initial point in the representation with a modified point, where the modified point has a larger size than the initial point (and where the modified point comprises a pointer to an image). The ‘size’ of the point may relate to a number of angular brackets covered by the point. Equally, the size of the point may indicate (e.g. based on coordinate values) an area of the representation or scene that is covered by the image. The above description has primarily related to generating a two-dimensional immersive image based on an identified two-dimensional image. In practice, the above-described methods typically involve identifying a plurality of points of a plurality of three-dimensional representations that can be replaced with a video, which video is associated with this plurality of three-dimensional representations. A frame of the video can then be identified and combined with one of the three-dimensional representations based on a synchronisation between the video and the representations. Typically, a video (or a plurality of videos) can be encoded using video encoding technology; one or more encoded videos may then be transmitted alongside (but, e.g. separately to) the three-dimensional representations with which these videos are associated. Any features that have been described herein with reference to an image apply equally to a video, and vice-versa. In some embodiments, the two-dimensional image may be determined and / or used based on a position, location, or perspective of a viewer. For example, the two-dimensional image may be selected based on the perspective of a user and the point of the scene that is being faced by the user. Typically, the two-dimensional image is relatively quick to decode / render (as compared to the three-dimensional representation). Therefore, while the three-dimensional representation may need to be decoded prior to the viewing of the two-dimensional immersive image, it may be possible to decode the two-dimensional image or video more quickly so that, for example, the two-dimensional image or video may be streamed. In some embodiments, the two-dimensional image comprises a real-scene; for example, the two-dimensional image may comprise a stream of a real-time video. The decoding of the three-dimensional representation and the determination of (aspects of) the two-dimensional immersive representation based on the three-dimensional representation typically comprises a different procedure than the decoding of the two-dimensional representation. Therefore, the three-dimensional representation may be processed (e.g. decoded and / or converted to one or more two-dimensional objects) separately to the two-dimensional representation. In some embodiments, this may comprise the identified three-dimensional representation being processed using different hardware to the two-dimensional representation, where different hardware and / or software may be suitable for these different processing procedures. For example, a X265 hardware block may be used to process (e.g. decode) the two-dimensional image while a computer processing unit (CPU) or a graphical processing unit (GPU) may be used to process the three-dimensional representation. The rendering of the two-dimensional immersive image depends on a viewpoint of the viewer of the scene. In some embodiments, the two-dimensional image comprises a tiled image, where only the tiles within view of a user are decoded and / or rendered at any given time. Such rendering based on a current viewpoint of a user is typically more plausible when using a two-dimensional image, since other tiles of the two-dimensional image can be readily rendered if the viewpoint of the user changes (whereas rendering objects from the three-dimensional representation typically requires more time, and therefore, objects in the three-dimensional representation that could be viewed by a user are typically pre-rendered so as to be ready for display on the display device 17 even if they are not visible from a current viewpoint of the user). In some embodiments, the two-dimensional background image (and / or video) is associated with a reference point. Similarly, the three-dimensional representation may be associated with a reference point. This ensures that the two-dimensional background video and the three-dimensional representation can be synced before the generation of the two-dimensional immersive image. A user may then be able to move the reference point within the video as part of modifying the three-dimensional representation. The reference point may be included within the point data of a background point where a computer device (e.g. the display device 17) is then able to render the two-dimensional background image (or video) based on a time defined by the reference point. Formation of a bitstream The processing of a three-dimensional representation that has been described above results in a processed three-dimensional representation that may then be stored and / or transmitted by a computer device. In particular, the processed three-dimensional representation may be encoded in a bitstream that is transmitted to another device and / or the processed three-dimensional representation may be used to render one or more two-dimensional images, which images may be encoded in a bitstream that is transmitted to another device. This bitstream can then be decoded by this other device in order to extract the processed three-dimensional representation and / or the two-dimensional image(s) from the bitstream. The present disclosure envisages a bitstream that contains or references a background video, where this background video is associated with one or more background points of the three-dimensional representation. For example, the bitstream may comprise a first section that defines a plurality of points of a three-dimensional representation (including one or more background points) and a second section that defines, or references, on or more two-dimensional background images that are associated with these points and / or that reference video data associated with these points. Figure 13 shows a schematic of such a bitstream comprising two sections, where each section comprises one or more bits. Bit-a to Bit-d forms a first section of the bitstream that signals one or more points of a three-dimensional representation and / or that defines one or more immersive two-dimensional images. Bit-e to Bit-f forms a second section of the bitstream that references or defines one or more two-dimensional background images (or videos) that are referenced by the points of the three-dimensional representation. These two sections of the bitstream may be differently encoded or decoded. For example, the second section may be encoded / decoded using traditional two-dimensional video coding techniques (such as HEVC, VVC, and / or LCEVC techniques), while the first section may be encoded / decoded using a three-dimensional encoding / decoding technique. While the bitstream is typically encoded in the order of the sections provided above, the bitstream may be encoded in any order. In some embodiments bits from the first section and the second section may be ‘interlaced’, where any background point that is signalled in the bitstream is immediately followed by a corresponding background video. The bitstream described above may be decoded by a decoding device and this may allow the original (or similar to the original) plurality of images to be re-generated. A method of decoding said bitstream may comprise the steps of: identifying a first and a second section of bits in a bitstream; generating a plurality of initial two-dimensional immersive images based on the bits in the first section of the bitstream; generating one or more two-dimensional background images based on the bits in the second section of the bitstream; and combining the initial immersive and background images to form one or more final two-dimensional immersive images. In some embodiments, the aforementioned sections of a bitstream are arranged to be decoded separately (e.g. by separate computer devices or processing units), where this enables a parallelised method of decoding the bitstream so as to speed up a process of decoding and rendering a scene. For example, each pair of background points and background images may be decoded and processed separately. In some embodiments, the bitstream comprises one or more flags that indicate features of the bitstream and / or of the three-dimensional representations or two-dimensional images signalled by the bitstream. For example, the bitstream may comprise one or more flags that indicate: whether the three-dimensional representation contains any background points (e.g. references any two-dimensional background images); a feature, e.g. resolution ora size, of the two-dimensional background images; a location of a repository that contains the two-dimensional images referenced by the background points (e.g. the repository may be the second section of the bitstream or may be a separate repository); and a process by which the two-dimensional background images should be combined with the initial two-dimensional immersive images (e.g. whetherthey should be provided behind the other points ofthe immersive images and / or whether they should be presented in layers within the immersive images). Alternatives and modifications It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope ofthe invention. The representation is typically arranged to provide an extended reality (XR) experience (e.g. a representation that is useable to render a XR video). The term extended reality (XR) covers each of virtual reality (VR), augmented reality (AR), and mixed reality (MR) and it will be appreciated that the disclosures herein are applicable to any of these technologies. The representation may be encoded into, and / or transmitted using, a bitstream, which bitstream typically comprises point data for one or more points ofthe three-dimensional representation. The point data may be compressed or encoded to form the bitstream. The bitstream may then be transmitted between devices before being decoded at a receiving device so that this receiving device can determine the point data and reform the three-dimensional representation (or form one or more two-dimensional images based on this three-dimensional representation). In particular, the encoder 13 may be arranged to encode (e.g. one or more points of) the three-dimensional representation in order to form the bitstream and the decoder 14 may be arranged to decode the bitstream to generate the one or more two-dimensional images. In some embodiments, the scene comprises a static scene; alternatively, in some embodiments the scene comprises a video and / or a moving (e.g. non-static) scene. That is, in some embodiments the scene comprises a static scene, such as a building, where a viewer is able to move through this scene, e.g. to view different rooms ofthe building, but where the scene itself does not change. In some embodiments, the scene comprises a moving scene, where elements of the scene vary in time even where the viewer remains stationary. It will be appreciated that typically the scene comprises both static and moving elements where, for example, non-static elements move in front of a static background. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope ofthe claims.

Claims

1. A method of processing a three-dimensional representation of a scene, the method comprising: identifying a three-dimensional representation of a scene;identifying one or more points in the three-dimensional representation as background points based on a distance of said points from a viewing zone associated with the representation;processing the identified points, preferably so as to associate these points with a two-dimensional background image.

2. The method of any preceding claim, wherein the two-dimensional background image comprises a monoscopic image.

3. The method of any preceding claim, comprising generating a two-dimensional immersive image based on the three-dimensional representation and the two-dimensional background image, preferably comprising generating a two-dimensional immersive image for each eye of a viewer, more preferably wherein each of the two-dimensional immersive images is based on the same two-dimensional background image.

4. The method of any preceding claim, comprising:determining a distance the identified points from a viewing zone associated with the scene; and associating the points with a monoscopic two-dimensional image in dependence on the distance exceeding a threshold distance; and / or;associating the points with a stereoscopic two-dimensional image in dependence on the distance not exceeding a threshold distance.

5. The method of any preceding claim, wherein processing the identified points comprises one or more of:removing the points from the three-dimensional representation;modifying the three-dimensional representation to include a reference to the background image; replacing an attribute value of the identified points with a reference to the background image; and generating the two-dimensional background image based on the identified points.

6. The method of any preceding claim, wherein the two-dimensional background image comprises a background of the scene and / or wherein the two-dimensional background image comprise a 360 degree image.

7. The method of any preceding claim, comprising:determining a distance of the identified points from a viewing zone associated with the scene; and determining the two-dimensional background image based on the distance;preferably, determining a resolution of the two-dimensional background image based on the distance.

8. The method of any preceding claim, comprising one or more of:rendering a two-dimensional image based on the three-dimensional representation and combining the rendered two-dimensional image with the background image based on the locations of the background points;forming the two-dimensional background image based on attribute values of the identified points; preferably, comprising forming the two-dimensional background image at a first device and transmitting the two-dimensional background image to a second device; andreceiving the background image and the three-dimensional representation from different databases and / or computer devices.

9. The method of any preceding claim, comprising:identifying a plurality of two-dimensional background images associated with the scene; and determining a two-dimensional immersive image based on the plurality of identified two-dimensional background images.

10. The method of any preceding claim, wherein determining the two-dimensional immersive image comprises:rendering one or more two-dimensional objects based on the three-dimensional representation; andsuperimposing the two-dimensional objects onto the two-dimensional background image.

11. The method of any preceding claim, wherein:the two-dimensional background image is associated with a location in the three-dimensional representation; and / orthe two-dimensional background image is associated with one or more points of the three-dimensional representation, preferably a location of said one or more points.

12. The method of any preceding claim, comprising:identifying the one or more points of the three-dimensional representation based on a characteristic of said points, preferably based on a distance of said points from a viewing zone of the scene; andcombining the two-dimensional image with the three-dimensional representation based on the identified points, preferably locating the two-dimensional image within the scene based on the locations of the identified points.

13. The method of any preceding claim, comprising:identifying a point of the three-dimensional representation;identifying a characteristic of the point; andmodifying the point based on the characteristic;preferably, wherein modifying the point comprises replacing the point with a two-dimensional background image and / or a reference to a two-dimensional background image.

14. A method of processing a three-dimensional representation of a scene, the method comprising:identifying a point of the three-dimensional representation;identifying a characteristic of the point; andmodifying the point based on the characteristic;wherein modifying the point comprises replacing the point with a two-dimensional background image and / or a reference to a two-dimensional background image.

15. The method of claim 13 or 14, wherein:the characteristic comprises a distance of the point from a viewing zone of the scene, preferably wherein the method comprises replacing the point based on the distance exceeding a distance threshold; and / orthe characteristic comprises a difference between a left eye attribute of the point and a right eye attribute of the point; and / orthe characteristic is associated with the point being static, preferably wherein the method comprises replacing the point based on an indicator of the point being static exceeding a threshold value.

16. The method of claim 15, comprising:identifying a first point in a three-dimensional representation of a scene;identifying a second point in a second three-dimensional representation of a scene; anddetermining that the first point and / orthe second point is a static point based on the locations and / or attributes of the first point and the second point;preferably comprising:determining that the first point and / or the second point is a static point based on one or more of: determining that the first point and the second point have similar locations; and determining that the first point and the second point have similar attributes; and / ordetermining that the first point and / orthe second point is a static point based on the locations and / or attributes of corresponding points in a plurality of three-dimensional representations, preferably at least 3 representations, at least 5 representations, at least 10 representations, and / or at least 20 representations.with a frame of a video.

17. The method of any preceding claim, comprising:processing the three-dimensional representation using a first hardware device; and processing the two-dimensional background image using a second hardware device.

18. A method of processing a three-dimensional representation of a scene, the method comprising: identifying a three-dimensional representation of a scene;determining a two-dimensional background image associated with the three-dimensional representation of the scene; andgenerating a two-dimensional immersive image based on the three-dimensional representation and the two-dimensional background image.

19. The method of any preceding claim, comprising generating a two-dimensional immersive image for each eye of a viewer, preferably comprising rendering the two-dimensional background image based on a viewpoint of a viewer of the immersive image, preferably wherein the two-dimensional background image comprises a tiled image and the method comprises rendering a tile of the two-dimensional background image based on a viewpoint of a viewer of the immersive image.

20. The method of any preceding claim, wherein determining the two-dimensional background image comprises one or more of:determining the two-dimensional background image based on a background point of the three-dimensional representation, preferably based on an attribute value of the background point; andextracting the two-dimensional background image from a bitstream containing the three-dimensional representation.

21. The method of any preceding claim, wherein the three-dimensional representation is associated with a viewing zone, the viewing zone comprising a subset of the scene and / orthe viewing zone enabling a user to move through a subset of the scene, preferably wherein:the user is able to move within the viewing zone with six degrees of freedom (6DoF); and / orthe viewing zone has a volume of less than 50% of the volume of the scene, less than 20% of the volume of the scene, and / or less than 10% ofthe volume of the scene; and / orthe viewing zone has, or is associated with, a volume, preferably a real-world volume, of less than five cubic metres (5m3), less than one cubic metre (1 m3), less than one-tenth of a cubic metre (0.1m3) and / or less than one-hundredth of a cubic metre (0.01m3).

22. A bitstream comprising one or more points processed using the method of any preceding claim.

23. A bitstream associated with a three-dimensional representation of a scene, the bitstream comprising one or more background points of the three-dimensional representation, the background points each being associated a two-dimensional background image.

24. The bitstream of claim 22 or 23, comprising one or more flags that indicate:whether the three-dimensional representation contains one or more background points;a feature, e.g. resolution or a size, of a two-dimensional background image associated with one or more background points;a location of a repository that contains a two-dimensional background image; anda process by which the two-dimensional background images should be combined with the initial two-dimensional immersive images.

25. The bitstream of any of claims 22 to 24, comprising one or more of:the two-dimensional background image; anda first section associated with the three-dimensional representation; and a second section associated with one or more two-dimensional background images;bits signalling two-dimensional background images, said bits being interlaced with bits signalling points of the three-dimensional representation.

Images